1. Field of the Invention
The present invention relates to a semiconductor laser and a fabrication method thereof and more particularly, to a semiconductor laser having a mesa stripe and an AlInP or AlGaInP burying layer that buries the mesa stripe at its both sides, and a fabrication method thereof by Metal-Organic Vapor Phase Epitaxy (MOVPE).
2. Description of the Prior Art
A 600 nm-band AlGaInP-system semiconductor laser, the lasing light of which is red, plays an important role as a light source for optical disks such as a compact disk (CD), a magneto-optical (MO) disk.
The semiconductor laser of this sort typically has such a structure as shown in FIG. 1 and is fabricated by MOVPE. This structure is disclosed in Electronics Letters, Vol. 23, No. 24, Nov. 1987, PP1327.
In FIG. 1, an n-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 22 is formed on an n-GaAs substrate 21. An undoped AlGaInP/GaInP active layer 23 with a Quantum Well (QW) structure is formed on the n-cladding layer 22.
A p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P inner cladding layer 24 is formed on the active layer 23. A p-Ga.sub.0.5 In.sub.0.5 P etching stop layer 25 is formed on the inner cladding layer 24.
A p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P outer cladding layer 26 is formed on the etching stop layer 25. An undoped Ga.sub.0.5 In.sub.0.5 P buffer layer 27 is formed on the outer cladding layer 26. A p-GaAs cap layer 28 is formed on the buffer layer 27. These three layers 26, 27 and 28 constitute a mesa stripe.
An n-GaAs current-blocking layer or burying layer 30 is selectively formed on the etching stop layer 25 at both sides of the mesa stripe to bury the mesa stripe therebetween.
A p-GaAs contact layer 29 is formed on the burying layer 30 and the uncovered cap layer 28.
With this conventional semiconductor laser, the current-blocking or burying layer 30 is narrower in energy band gap than the active layer 23. Therefore, there is a problem that energy loss increases in the cavity or resonator of the laser because the lasing light is absorbed by the layer 30, decreasing the external quantum efficiency.
To solve this problem, another conventional semiconductor laser shown in FIG. 2 has been developed, which is termed a real-index guided laser. In this laser, an AlInP or AlGaInP layer that is wider in energy band gap than the AlGaInP/GaInP QW active layer 23 is employed as the current-blocking or burying layer 30.
Such the laser is, for example, disclosed in the Japanese Non-Examined Patent Publication No. 4-154183 (May, 1992).
The conventional real-index guided laser shown in FIG. 2 has the same structure as that of the laser shown in FIG. 1 excepting that an n-AlInP current-blocking layer 31 and an n-GaAs cap layer 32 are formed instead of the n-GaAs current-blocking layer 30.
The n-AlInP current-blocking layer 31 is selectively formed on the etching stop layer 25 at both sides of the mesa stripe to cover the respective side faces of the mesa stripe and the uncovered surface of the etching stop layer 25. The layer 31 is made of first regions 31a that are contacted with and extend along the respective side faces of the mesa stripe, and second regions 31b that are joined with respective bottom ends of said first regions and extend along the surface of the etching stop layer 25.
The n-GaAs cap layer 32 is selectively formed on the current-blocking layer 31 at the both sides of the mesa stripe to bury the mesa stripe and the current-blocking layer 31.
The p-GaAs contact layer 29 is formed to be contacted with the cap layer 28, the current-blocking layer 31 and the n-GaAs cap layer 32.
In fabrication of the conventional real-index guided semiconductor laser of FIG. 2, typically, the growth conditions such as the growth temperature and the supply rates of source materials for the above semiconductor layers are set so that Al.sub.x In.sub.1-x P or (Al.sub.y Ga.sub.1-y).sub.x In.sub.1-x P is lattice-matched to the (001)-plane of GaAs, in other words, the composition x is equal to 0.5 (i.e., x=0.5).
In this case, however, the first regions 31a contacted with the side faces of the mesa stripe, each of which is formed of the (111)-plane, are grown to be In-excessive, i.e., x&lt;0.5, because in atoms are more readily incorporated into the lattice sites than Al atoms.
Accordingly, some lattice strain occurs due to lattice-mismatch, so that sudden deterioration of the laser tends to take place. This is a serious problem relating to the operation reliability of the laser.
On the other hand, a semiconductor material containing Al such as AlGaAs and AlInP is difficult to be grown selectively at the both sides of the mesa stripe by the popular MOVPE techniques. However, a selective growth method of AlGaAs has been realized by adding hydrogen chrolide (HCl) during the growth process of the Al-containing semiconductor material, which is disclosed in Journal of Crystal Growth, Vol. 124, 1992, pp235-242.
In this selective growth method, the first regions 31a contacted with the side faces of the mesa stripe are grown to be Ga-excessive, because Ga atoms are more readily incorporated into the lattice sites than Al atoms. This means that the first regions 31a are different in composition from the second regions 31b.
With AlGaAs, since the composition dependency of the lattice constant is comparatively small, there is no possibility of the lattice strain in spite of the composition difference between the first regions 31a and the second regions 31b.
On the contrary, with AlInP and AlGaInP, since the composition dependency of the lattice constant is comparatively large, the lattice strain due to the composition difference or lattice-mismatch tends to take place. This is a serious problem relating to fabrication and operation reliability of the laser.